84 VF1000F seems to start cutting fuel at 9k RPM

I’ve been restoring my bike recently and noticed the redline seems to have moved down to 9-9.5K. After that it feels as if it starts to cut fuel and tops out before 10K. Anyone else have this problem or know what the issue could be? Thanks in advance.

Hi zafarack…
The carb float highs are critical on the vf… if the hights are set too big it will run lean at the top end of the rev range…from memory I think they should be 7.5mm

That could totally be my problem… I opened them up to clean the jets and all and don’t think I adjusted them correctly back. Will report back as soon as I get a chance to open them up! One other thing I was thinking is that there could also be some rust or bloackage in my fuel pump or filter… Could that cause this issue? Thanks so much for the advice, defintely would not have checked my floats if it wasn’t for you!

it would certainly be worth testing your fuel pump, there is a procedure in the manual but basically, pull the fuel line apart just before it goes into the carbs, put the fuel hose end into a measuring jug, then disconnect the multi-plug which goes into the fuel pump relay, bridge the black and white wires in the multi-plug with a piece of suitable wire, switch the ignition on and the fuel pump should start to run, it should pump 800cc in one minute.

for what the fuel filters cost these days it would probably be worth changing it if you have any doubts.

Thanks again! I took out my fuel filter this morning and there was definitely quite a bit of rust/build up in them, so I went ahead and cleaned it and reinstalled it to the bike. Quick question: is cleaning it okay or should I just go ahead and buy a new one? It looks good now and seems to run well. Definitely getting a lot more power now, and seems to rev a little higher. Going to (hopefully) check the carb floats tonight to get it perfect. One other thing I was thinking may need attention are my air filters - the previous owner pulled off the air box and stuck 4 K&N filters in its place, so I’m going to make sure none are blocked in any way that could impede air flow. Appreciate all the help. Will report back with my findings after I take a look at the carbs!

Hi,a filter is a service item so does need replacing from time to time.By the sound of things now is a good time.
Pete has covered a few possible causes for the symptoms you describe.
I had a simular problem where the bike ran perfectly to 7000 rpm but no higher.
All jets were clean and there were no holes in the diaphram`s only after a session in an ultrasonic bath did it run properly.
Does operating the choke at 9k make a difference?If it picks up it would suggest an internal blockage if not a problem with supply to the carbs.
If it is a weak mixture problem choked air filters would help rather than hinder but with the setup you have it sounds like the carbs have access to way too much air unless that is they are jetted to suit.
Regards Bif

The bike is actually running great all the way up to 10.5k now. Cleaning the filter and the gas tank seemed to do the trick (though I will still replace the filter when the new one comes). The only issue is that when you quickly turn the throttle and hold it wide open, it stutters around 5-6k, then goes up, stutters at 8k, and then goes up again. Any idea what could cause this? Would this be related to the carb floats? Thanks again for your help :slight_smile: Really appreciate it!

Ok, I was finally able to get a tool to measures the floats precisely and i checked one so far. It was 8.25mm vs spec of 7.5mm. Before I go messing with the tangs and adjust the floats (assuming I measured it right), I just wanted to verify that a higher float number could indeed cause the symptoms I observe. It makes sense to me, that the floats would then close sooner causing less fuel flow but just need someone to verify for me that this is correct. I’ll wait to make the adjustments till someone gets back or in a day I’ll just go ahead and see what I get. Also there is some yellowish tinge in the bowls. I’ve been using some fuel injector cleaner mix in the fuel like techron and some murphy’s mystery oil. I’ve seen rust before and it looked more reddish, this is more like turmeric like color. I’ll clean them before putting them back but if anyone knows what that color could indicate please let me know. Thanks again!

You are correct,the bigger the number the faster the valve closes and the lower the fuel level in the bowl.
The residue in the float bowl is lacquer from stale fuel.Fuel system cleaner may help,but if you have residue in the bowl there may well be some in the internal passageways of the carb
Regards Bif

Ok, adjusted the floats. Carb 2 was pretty near spec and others were at 8.2-8.35mm. I adjusted all of them to spec of 7.5mm. Cleaned all the carbs, removed the lacquer (BTW #2 was clean as a whistle). As it has been raining I did not get to do any test runs for a while. However, I was able to do a test run today and its still the same.

So I removed the air filters and went out for a ride. The bike was cold by now and the temp had gone from about 97F (36C) to 75F (24C) due to a storm coming in and rain. I rode it after the storm and when the roads were dry and it seems without the air filters (4 K&N cones) the bike seemed to go much quicker through the bumps and then peaked at 10-10.5K. As the bike warmed up the slight bumps at 5K and now around 9K (sometimes there would be no bump at 9K and sometimes there would be but it always occurred around 5K) would become more noticeable.

I have not checked the fuel pump yet. Will do so as soon as I can. The setup is Kerker exhaust and K&N cone filters on each of the air funnels. The reason I removed the filters was not because they were dirty, but because I think the gas tank sits on the ones going into #2 and #4 (straight up) and pushes them down where the funnels have to be inserted higher up into the filter and so I think it reduces air flow. removing them certainly made it seem much more responsive and at first couple of runs I thought it had fixed the problem. But I think as the bike really warmed up, its performance went down a bit but much better than before.

So next up, test the fuel pump. If its ok, then I think the main jet may need adjustment and I’m going to have to get shorter filters for #2/4. The previous owner said he had rejetted it for the free flow exhaust and the K&N filters and I had the jets out but there were no numbers on them that I could see. I’ll pull them again and look once more and be sure to measure them with my vernier caliper. Let me know if I should look elsewhere or do some other tests. Otherwise I’ll post up what I find next. Thanks again, and sorry for the long post :slight_smile:

hi zaffarack,

you may have answered your own question, maybe the warmer air that is created between the cylinders is causing the flat spots, most engine’s seem to like cool damp air, it would be interesting to see if fitting the original air box made any difference

Yeah, I thought of that, which is why I posted the temp data and the fact that it was damp, dense air. However I am in want of answers so I did not want to take my own hunch and assume an easy solution. But glad that you also thought of the same thing and confirmed my belief.

However we have a problem. The original airbox and cover are nowhere to be found and I’d also have to get new air funnels that fit the airbox.

So I did an easy test. Ran without the filters and top out somewhere around 10-10.5K. Put the air filters on and top out at 9.5K + some slack depending on how I did the throttle from the start (hard to keep the throttle opening the same way as the rocket is flying and you know that I can’t kamakazi all over the roads :-). I’d say a diff of about 0.5K give or take.

I am sticking to some isolated back roads but I still have to watch for rodents, leaves, wet spots, pot holes and occasional cars and the like. The machine is still, even by today’s standards, an absolute beast! Wish I had an abandoned airport nearby!

So at this stage I think I will go forward. When I’m all done I will write up all that I’ve done. It’s been months of labor and painstakingly putting everything together sourcing some hard to find parts and such. I want to finish this and make it perfect.

So I’m thinking of checking the jet sizes. I don’t know how but I’ll learn. Then we will know a bit more and see if going up in size on the main jet is a good idea.

All this takes time and I’m stealing hours here and there to get to the end. If anyone thinks I’m on the wrong track, stop me now!

I actually found an old picture of my 3rd carb, and the main jet has “160” written on it. I did a calculation on one of the web sites for a VF1000F with pods/cones and a baffled exhaust or a non-baffled exhaust and they come up with 155, and 158 respectively (are these websites accurate enough to tell me what jets to pick or is there some other way of knowing?). Do you guys think that a 160 main jet, if a bit too large, could cause the effect I’m observing? My guess is that if the jets are just a bit too large they could cause not enough fuel to be sucked up by the airflow (Venturi principle) and hence the bike runs lean at WOT. Thanks again!

EDIT: I’ve actually checked some things out and it seems like the main jet size could be causing my bike to run rich & sputter a bit at the higher RPMs. I’m guessing this is because the previous owner changed out the jets but did not get the correct size to go with my Kerker (w/baffle) exhaust. I am thinking of running the bike without the exhaust on just to see if that frees up air flow and more closesly matches the 160 jets on it. If that is the case, I will remove the baffle from the exhaust and find the correct jet and needle size to go with that setup. What do you guys think of that idea?

Difficult to do a good set up by feel alone,a run on a dyno would give accurate and repeteable data.
From the start I made the mistake of thinking the bike was standard but with the intake and exhaust as they are and not knowing if the carbs have been fitted with a Dynojet kit its beyond me.
Regards Bif

Hi there in my opinion it is almost impossible to fit after market cone filters and just up jet it and expect these vacume carbs to behave properly the air boxes on these bikes are designed to run slight pressure removing it and then up jetting the carbs by 10% and putting cone filters on will not do it any good you will first notice a hesitation at about 4 to 6000 rpm then it will pull for about 3000 rpm before starting to hold back again the only way to get these carbs to work properly would either return them back to standard or fit a stage 1 dyno kit I dont think these are available for the vf that is why the cone filters wont work cone filters will only work on the earlier carbs where the throttle slide is controlled by cables and not vacume air pressure like your carbs that is why dyno jet kits were invented as the other way does not work i hope this helps and please dont run the bike flat out with no exhaust it will cause hotspots to develop on the exhaust valves and weaken their structure i would first return the bike to standard and then try it, you can then decide if there are any other faults

I actually opened up my carbs and took a look at the jet needles. The previous owner had put a few washers on it and this was causing the bike to run rich at 5k. After removing the washers and test riding the bike, I can now say that the mid range is perfect! However, with the air filters on, it still only revs to about 10K. I’ve read that a smaller main jet should fix this issue, so I am going to order one and check that out. Thanks again for all your help, I will keep you posted. And Bif, sorry I didn’t specify the modifications to the bike earlier, I actually was not sure what the previous owner did to it as we bought it from him a long time ago :slight_smile: Unfortunately, I don’t have access to a dyno… But I certainly wish I did! So far I’ve been able to just feel it out, so wish me luck doing that lol. Appreciate the help though!

Just an update for anyone following along or has the same issue in the future: I sized down my main jets today (from 160 to 155) and the bike runs absolutely perfect now. Revs all the way up smooth and incredibly quickly. Also tuned the air/fuel mixture screws to fix some popping from the exhaust, and will do a carb sync soon to get it 100%. Thanks for all the help guys, I really appreciate it!